On 25 Jan 2006, at 21:11, Ed Barkmeyer wrote:
> I need to understand what FranÃ§ois has in mind by "expression in a
> query language". I can understand how something like:
> (SPARQL 'RDFbase '<query text> <rule-expression> ... <rule-
> expression>)
> could work as an "external query", where:
> - SPARQL designates a particular query service (or a class of query
> service for which the rule engine is to find a server in
> conjunction with the "RDFbase" parameter)
> - RDFbase designates the KB on which the service is to operate for
> this query
> - <query text> is a query stated in the language of the designated
> service that may contain references to "external parameters" using
> the "external parameter syntax" *for that query language*
> - <rule-expression> is an operand expression in the rule language
> (RIF) syntax. The expression is evaluated by the rule engine
> before invocation of the external query service, and the result of
> the evaluation is passed in the position in which the expression
> occurs, i.e. an "actual parameter".
I am sure (or hope) that what are you talking about is only *one*
option out of the *many* that RIF has to characterise and define in
order to interoperate with ontology languages (such as RDF, in this
example): let's call this option the 'trivial'semantics.
Why am I saying 'ontology languages'?
What you call an "expression in a query language" is in the very
general case an open formula in some ontology/knowledge-
representation language, whose bindings make the formula 'somehow'
formally connected with some knowledge base (the RDFbase in your
example) -- in your case the connection is the bare logical
implication. I guess that this is a fair (informal) formalisation of
this 'trivial semantics'.
However, there may be several kinds of these 'connections': most of
them are based on a model-theoretic characterisation rather than on
entailment (see [1] - and I can really think of at least three
additional important classes: FOL semantics (Ã la SWRL), LP-weak-safe
semantics (Ã la Rosati), and autoepistemic semantics). So, we are
really talking about at least 4 different semantic options to
characterise the interoperability between a knowledge base and rules
by means of 'query expressions' (as defined above) appearing in the
body of some rules.
To be more concrete, let me take a very special case, and see how it
behaves.
Let us restrict attention to the RDF and OWL ontology/knowledge-
representation languages (we have at least to consider those two, as
per our charter). In order to super-simplify our life, let us in
addition restrict our attention to the case when those queries are
atomic: atomic binary predicates (a triple for RDF, a role for OWL)
and atomic unary predicates (a class in OWL).
By adopting the 'trivial' semantics above, it is impossible to
correctly capture correctly, for example, the function-free horn
clause fragment of SWRL (which is, if you think a little about it, a
special case of the above but with FOL semantics); basically none
(but one) of the approaches surveyed in [1] would be captured
correctly; and 100% of the use cases in <http://www.w3.org/2005/rules/
wg/wiki/Managing_incomplete_information>, while perfectly expressible
in the trivial setting, would miserably fail.
So, while I believe that there should be room for the 'trivial'
semantics above, RIF should try to characterise also the several
alternative approaches as known in the literature.
cheers
--e.
Enrico Franconi - franconi@inf.unibz.it
Free University of Bozen-Bolzano - http://www.inf.unibz.it/~franconi/
Faculty of Computer Science - Phone: (+39) 0471-016-120
I-39100 Bozen-Bolzano BZ, Italy - Fax: (+39) 0471-016-129
[1] Enrico Franconi and Sergio Tessaris (2004). Rules and Queries
with Ontologies: a Unified Logical Framework. Workshop on Principles
and Practice of Semantic Web Reasoning (PPSWR'04).
<http://www.inf.unibz.it/%7Efranconi/papers/ppswr-04.pdf>